1. Field of the Invention
The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery comprising the same.
2. Description of the Related Art
Recently, interest in electrochemical devices is increasing day by day. As the electrochemical devices are extensively applied to mobile phones, camcorders, and notebook computers, and further to electric vehicles or hybrid electric vehicles, the demand for high energy densification is increasing in the field of electrochemical devices used as a power source of such electronic appliances. Lithium secondary batteries are the most suitable to meet the demand, and thus, their study is being made actively at present.
Among the currently available secondary batteries, lithium secondary batteries developed in the early 1990's comprise an anode of a carbon material capable of intercalating and disintercalating lithium ions, a cathode of lithium-containing oxide, and a non-aqueous electrolyte solution having a proper amount of lithium salt dissolved in an organic solvent.
The lithium secondary batteries generally have an average discharge voltage between about 3.6V to about 3.7V, and thus, they advantageously have a higher discharge voltage than alkali batteries, nickel-cadmium batteries, and the like. To exhibit such a high operating voltage, it needs an electrolyte solution composition which is electrochemically stable in a charge/discharge voltage range between 0 to 4.2V. For this purpose, a non-aqueous electrolyte solution uses, as an organic solvent, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate, propylene carbonate, and the like, is properly mixed with a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, and the like.
The organic solvent has a low flash point and a high degree of burning performance, which may affect safety of lithium secondary batteries. In particular, when batteries are exposed to high voltage conditions such as overcharge caused by malfunction of a charger or carelessness of a user, the organic solvent gets in contact with a cathode or an anode and it decomposes, resulting in a heat generating reaction accompanying gas generation. As a result, batteries may have an increase in thickness, which may cause a problem during setup of mobile phones or notebook computers, or batteries may suffer from inner pressure increase and thermal runaway, resulting in explosion or firing.
To solve the above problems, non-aqueous electrolyte solutions containing a variety of additives have been suggested. For example, batteries including an electrolyte solution containing 4-Chlorotoluene (4CT) have been developed to reduce gas generation when overcharged. However, the exemplary additive was designated as an environmentally hazardous substance and impossible to commercialize. Accordingly, there is a need for an additive that replaces the conventional additive.
U.S. Pat. No. 7,223,502 discloses a non-aqueous electrolyte solution containing a variety of fluorine-containing aromatic compounds for suppression of gas generation. Also, Korean Patent Publication No. 2006-0029748 teaches a non-aqueous electrolyte solution containing various kinds of halogenated toluene for improving overcharge characteristics. These papers merely mention difluorotoluene as one of additives, however they do not suggest or specify remarkable effects of difluorotoluene for inhibition of decomposition of a non-aqueous electrolyte solvent.
Meanwhile, Korean Patent No. 0760763 discloses a non-aqueous electrolyte solution containing halogenated biphenyl and dehalogenated toluene for ensuring overcharge safety of a battery. However, this document does not suggest or specify remarkable effects of difluorotoluene for inhibition of decomposition of a non-aqueous electrolyte solvent. Like this art, when halogenated biphenyl and difluorotoluene are simultaneously added to a non-aqueous electrolyte solution, halogenated biphenyl having a lower oxidation potential first covers a cathode to form a thin film which hinders oxidation of difluorotoluene at the cathode. Accordingly, thin-film formation of difluorotoluene on an anode is hindered, and as a result, a function of difluorotoluene for preventing decomposition of an organic solvent is remarkably reduced when overcharged.